Small portable equipment such as mobile phones and Personal Digital Assistants (PDAs) have a limited-size keyboard, i.e. the keyboard has a smaller number of keys than a standard QWERTY-keyboard or a 104-key PC-keyboard. Of necessity a limited-size keyboard assigns multiple characters to each key.
FIG. 1a shows a mobile phone 100 having fifteen keys 101 arranged in three columns and five rows on its front surface. At least some of the keys are multi-tap keys. An individual multi-tap key has multiple contents, i.e. different characters are displayed depending on the number of taps made on the same multi-tap key.
The traditional way of inputting a word is that each letter in the word is input one at a time in a particular edit-word mode or a multi-tap text input mode. Hereafter, the multi-tap text input mode will simply be referred to as the multi-tap mode, which is used with mobile phones for entering what is termed a “short message” (SMS). In this mode a multi-tap key must be pressed one or more times until the desired number, letter, or other character or symbol is displayed.
FIG. 1b is an enlargement of a portion of the limited-size keyboard that is shown in FIG. 1a. 
The word TEXT is entered in the multi-tap mode (also called ABC mode) according to the multi-tap text entry technique in the following way. The letter T is input by pressing the numeric key 8 once, the letter E by pressing the numeric key 3 twice, the letter X by pressing the numeric key 9 twice, and the letter T is entered by pressing the numeric key 8 once. Digits are input in a similar way. For example, the digit sequence 5910 is entered so that the numeric key 5 is pressed four times, key 9 is pressed five times, key 1 is pressed seven times, and key 0 is pressed two times. Thus the selection of a certain letter requires one to five consecutive taps on the same key, and the selection a certain number requires two to seven consecutive taps on the same key.
The keys are often also assigned a plurality of special characters. As an example, special characters in some European languages are as follows: Finnish and Swedish: å, ä, ö; Danish: å, æ, ø; and French: à, â, ç, è, ë, í, ì, ô, ù, û, ü, œ. The selection of a certain special character usually requires at least five key taps.
A major drawback with the above method is that the writing of a short message is slow, difficult, and frustrating.
In publication WO 01/22697 an accessory keyboard is presented as a solution for devices having a limited-size keyboard. The accessory keyboard comprises the QUERTY alphanumeric keyboard and a detachable connector which can be connected to the accessory connector of the portable radio telephone. This solution has several drawbacks. First, the accessory keyboard enlarges the size of the telephone. Second, one must remember to take along the accessory keyboard. Third, the acquisition of the accessory causes the user extra costs.
Predictive text input methods have been developed for entering text in equipment having a limited-size keyboard. The aim of these methods is to minimize the number of keystrokes required for entering text, i.e. to enable text entry using only one keystroke per key regardless of how many characters have been assigned to each key. The T9® text input method developed by Tegic Communications Inc. is one example of the predictive text input method. The idea of the method is described briefly in the following.
FIG. 1c–d illustrate how the T9 text input method works.
It is assumed that the T9 text input method, i.e. the predictive text input method, is initially active and that the Finnish dictionary is in use. The main blocks in FIG. 1c are multi-tap keys 101, the T9 analyzing system 103 including the T9 engine, the T9 database 104, a display 105, buffers (not shown), and the possibility of changing the T9 text input mode, hereafter termed the T9 mode, to the multi-tap mode (ABC mode) and vice versa.
In FIGS. 1c and 1d it is assumed that initially a buffer in the T9 text input mode is empty.
In the T9 mode the keystroke sequences are transformed into word predictions. Eight key taps are needed for a word having eight letters. Thus when a user wants to enter the Finnish word “kastella”, the numeric keys 5, 2, 7, 8, 3, 5, 5, and 2 are pressed once consecutively. The T9 text input system analyzes all the letters after each keystroke, and the T9 engine tries to create a word. The idea in the T9 text input method is that the user need not worry about what is displayed until the word is completed. In this example the word predicted after the completed keystroke sequence 5-2-7-8-3-5-5-2 102 is “järvellä”.
It is usual that more than one word shares the same keystroke sequence. Here the completed word differs from the desired word because the word “kastella” was not considered the most frequently used word.
However, if the word created is not the intended word, the user has the possibility of requesting a list containing other possible words, scrolling the list on the screen, and selecting the desired word by pressing the selection key. If the right word has not been deposited in the T9 database dictionary, it will also be missing from the choices in the list. In that case the word can be added to the database 104 manually by using the multi-tap text entry technique described above.
FIG. 1d is a table for illustrating in detail the keystrokes needed for the word “kastella”.
The first column shows the action, the second and third indicate the status of the multi-tap mode and the T9 mode, respectively, and the fourth column shows the content of the text buffer after each action. The content of the text buffer is also immediately displayed.
At the beginning the T9 mode is active (ON) and the multi-tap mode is inactive (OFF). The word “kastella” has eight letters which are input by pressing the keys shown in the first eight lines in the first column of the table. Because the completed word is wrong, the NEXT key is used to cycle through the other word options. In this case, the option is “kartellä”, which is against the Finnish phonetic rule of vowel harmony. Another tap of the NEXT key brings again the word “järvellä”. The word “kastella” is missing from the mobile dictionary, and it must be added using the multi-tap mode. In some cases the mode may change automatically from the T9 mode to the multi-tap mode, or alternatively the user changes the mode.
When the multi-tap mode is set as active (ON), the T9 mode is simultaneously set as inactive (OFF), i.e. the T9 mode does not predict the words.
One drawback is that the user must first delete the complete word in the text buffer, clearing it letter by letter from the end of the word. This requires eight keystrokes of the CLEAR key. After the T9 mode is changed to the multi-tap mode, the word must be added by pressing numeric keys in the following way: key 5 twice, so that the content of the text buffer is “k”; key 2 once, so that the content of the text buffer is “ka”; key 7 four times, so that the content of the text buffer is “kas”; key 8 once, so that the content of the text buffer is “kast”, key 3 twice, so that the content of the text buffer is “kaste”, key 5 three times, so that the content of the text buffer is “kastel”, key 5 three times, so that the content of the text buffer is “kastell” and key 2 once, whereupon the content of the text buffer is finally the desired word “kastella”. The word can now be saved in the dictionary, i.e. in the database 104, from which it can be retrieved in T9 mode when needed. After saving the word, the user must return to the T9 mode by pressing a mode change key. At least 35 keystrokes were needed to type the word “kastella” having 8 letters. This total does not include keystrokes needed for mode changes and saving.
It is clear that writing messages can be frustrating and slow if the text comprises several words needing an operation like the one described above. How easy it is to write with the T9 text input method also depends on the structure of the language used.
The structure of the Finnish language differs considerably from other European languages. It does not belong to the Indo-European language group to which Swedish, English, German, French, and Russian belong, for example. Typical to the Finnish language is possessive suffixes that are added to the end of a stem and an abundance of case endings and prefixes. Problems may arise especially when inflectional forms of Finnish are used with the T9 text input method.